The invention pertains generally to communication systems and is more particularly directed to communication systems used in conveying the values of a multiplicity of analog parameters from one point to another.
In large process control or monitoring systems there is a need to communicate the values of analog parameters from one point to another. In control systems, the values of parameters from sensors representing measured physical conditions of the process are communicated to the control and the values of parameters representing control variables are communicated to actuators. In many instances, the number of input variables and the number of control variables in such systems can be quite extensive. Because each input and output necessitates a connection, the cabling plant of a large multivariable system can grow rapidly to where it becomes somewhat unmanageable. In addition, when a distributed process is being controlled or monitored, the cabling plant from the remote sensors and actuators can become even more burdensome and expensive. The cost and complexity of the cabling plant grow directly with the number of sensed, monitored, or controlled devices and their distance from the processor of the system.
What is needed is a technique for concentrating the sensor and control information to reduce the cabling and connection costs of these systems. Many times in distributed processing, monitoring, and control systems multiplexers and local area networks are used. Such systems concentrate information from various sources into a digital format and transmit it on a communication link to the controller or monitor.
However, local area networks in many situations are not advantageous because the base control or monitoring system must be replaced along with the cabling. This is because the system protocols, network requirements and information rates of the LAN are incompatible with the base control or monitoring processor. For many installations where significant investments have been made in the base systems, such network solutions are untenable and may even be incompatible with the control or monitoring system requirements. Thus, there is a considerable need in these types of installations for a system which can concentrate information to reduce the cabling and connection costs of the sensors and actuators, but can also contemporaneously expand the information after the transmission so the information presented to a base control or monitor processor is compatible with the prior installation investment.
Such a system must also be extremely flexible in order to take into account the variety of different sensors and actuators which these systems may include. In general, many sensors provide either voltage source, current source, contact closure, or TTL type signals and most actuators can be controlled by these types of signals. Further, the system must be expandable to handle multiple inputs and outputs from distributed points and different sizes of systems. Such a system should include a communication protocol that is exact so that a multiplicity of variables can be accurately conveyed from the sensors and to the actuators. The system must also remain inexpensive and uncomplicated to meet the primary objective of relieving congestion of the cabling plant in such monitoring and control areas.